The OBJECTIVE of this proposal is to characterize the components of protein and calcium metabolism that are altered and to estimate their contribution to the overall protein and calcium requirements of Rett(RS) girls. The SPECIFIC AIMS for this project are:1)to determine if rates of body protein degradation, leucine oxidation, and urea production are higher and body protein synthesis, net protein retention, and the splanchnic extraction of lysine are lower in RS than in healthy girls, using a primed, constant infusion of [1-13C]leucine, [d4,4,5,5]lysine, and [15N2]urea to calculate rates of whole body protein synthesis, degradation, leucine oxidation, net protein retention, the splanchnic extraction of lysine, and urea production; 2)to determine if fractional rates of intestinal calcium absorption are lower and rates of diet- and bone-derived urinary calcium excretion are higher in RS than in healthy girls, using a single bolus dose of 42Ca and 46Ca to calculate fractional rates of calcium absorption, total absorption, net balance, and diet- and bone-derived urinary calcium; 3)to determine if serum insulin, IGF-1, growth hormone, and osteocalcin levels are lower and urinary cortisol outputs are higher in RS than in healthy girls.